Yes, as with many Rickettsiales bacteria, the organism can be harbored in the body for a very long time, but as long as the immune system is healthy, once the acute disease it brought into check with antibiotics, normally it can be kept in check and the dog remains symptom-free. She would only be a reservoir is a non-infected tick should latch on, gorge and drop off--that tick would then be infected--but I don't believe that casual contact would be dangerous for either you or your other animals. It does need a tick vector to be transmitted.
Was it an Ehrlichia or was it Anaplasma? (Anaplasma phagocytophilum
was just recently pulled out of the Ehrlichias, renamed and put into its own Genus by the taxonomists and some vets still call it Ehrlichia phagocytophilum
We have a lot of experience with Anaplasma and the treatment that seems to work the best for us is 4-6 weeks of doxycycline. Any less than that and you run the risk of relapse and having it become chronic. So far, we've not had any cases of chronic disease, probably because we're so hyper about it and are on the watch for any symptoms that we catch it very early. We do have one girl that came to us with chronic Lyme's, however. She does have occasional relapses that we treat with doxy and so far (knock on wood) we're always been able to get her back to near remission with the antibiotics.
She's very fortunate to have come to you! What a cutie and so glad that she's gaining weight--and probably feeling loads better since you started the doxy, too!
As for Frontline, we also use it. It kills most of the ticks, but when the tick population explodes, there's always the chance that one or more tough individuals will make it through the Frontline. We check the dogs every day, but that doesn't mean you can always find the ticks till they've been on long enough to swell a bit...and that means they've been on long enough to transmit disease.
It pays to be very vigilant for symptoms--mild lethargy, rotating lameness, increased drinking/urination, no matter how slight.
along for your little darling!!!